1. Field of the Invention
The present invention is directed to riser strings used with offshore rigs and vessels, keel joints used in various well operations, and to methods of their use.
2. Description of Related Art
Wellbore operations from floating vessels typically utilize risers or tendons in a string that extends from the vessel to the sea floor. Such floating vessels include tension buoyant towers, compliant towers, and spars in which the structures extend well below the sea surface and are subjected to heave, pitch, and roll motion at the surface. The risers and tendons are connected to the sea floor and pass through openings in the keel or bottom portion of the vessels. The openings in the vessels constrain the pipe forming the risers or tendons when the vessel is moved laterally with respect to the sea floor connection.
In many prior systems a special section of riser called a “keel joint” is used adjacent the keel of the hull: to accommodate the bending loads where the riser leaves the support of the platform; to accommodate the relative vertical movement between the riser and the hull; and to protect the riser string and the hull from damage. Several prior keel joints are reinforced to carry the bending loads imposed on the riser by the pitch/heel motions of the hull relative to the riser as well as the bearing and wear loads imposed on the riser by the vertical and lateral motions of the hull relative to the riser. Lateral movement can produce bending of the riser pipe at the hull opening, or rotation of the riser pipe about the contact of the riser pipe with the edges of the opening. Bending of the pipe, which is normally under tension, can result in fatigue and wear at the opening.
The prior art includes a variety of patents directed to vessels, risers and keel joints, including, but not limited to, U.S. Pat. Nos. 4,634,314; 5,377,763; 5,628,586; 5,683,205; 6,422,791; 6,739,395; 6,746,182; 7,013,824; 7,144,048; and 7,217,067—all incorporated fully herein for all purposes. Certain prior systems include the use of thick-walled pipes with tapered ends. These thick, tapered wall sections have been machined from heavy forgings and can be relatively expensive. Another solution utilizes a sleeve member centralized within the vessel opening and a mud line or sea floor connection to receive the lower end of the pipe. The pipe is centralized within the sleeve but otherwise unattached to the sleeve. Other approaches use a centralizing, ring-like device and/or a ball joint, located between the side walls of the vessel opening and the pipe.
U.S. Pat. No. 5,683,205 discloses a joint that passes through the vessel opening and is connected to the sea floor with the pipe centralized within an outer sleeve with large elastomeric rings located at each axial end of the sleeve. U.S. Pat. No. 6,422,791 discloses a sleeve positioned around a riser pipe where the pipe penetrates the keel of the platform. The riser-to-sleeve attachment provides a load carrying capacity in both the axial and lateral direction (or reduced capacity in one of these directions) and permits flexibility for angular offsets between an outer sleeve and a riser pipe.
U.S. Pat. No. 7,217,067 discloses a riser joint keel assembly in which a tapered riser joint is connected to a larger diameter outer sleeve through a connection that allows the tapered section and outer sleeve to function as one unit. In the combined design, the outer sleeve provides the required sliding interface between the riser and the guide at the keel of the hull while also providing some of the bending compliance needed to transition from the riser supported in the hull to the riser unsupported below the hull. The tapered section also provides the remaining bending compliance needed for the transition. The connection between the tapered and sleeve sections is a forged, machined ring plate welded to the bottom end of the sleeve, which provides a base for either bolted or threaded type attachment points for the tapered riser joint below the ring plate and the internal riser joint that continues to the surface. In one aspect such an assembly for a floating offshore structure with a top-tensioned riser arrangement, includes a compliant riser keel joint assembly, including: a. an outer sleeve positioned inside two keel guides in the hull structure; b. an internal riser joint positioned in said sleeve and having a flange attached at the lower end; c. a centralizer mounted inside said outer sleeve adjacent the upper end and sized to receive said internal riser joint; d. a single, tapered riser joint positioned below said internal riser joint; and e. means for connecting the lower end of said internal riser joint to the upper end of said single-tapered riser joint, including i. said internal riser joint having a threaded lower end; ii. a flange attached to the upper end of said tapered riser joint; and iii. a machined ring attached to said sleeve, said ring having a threaded bore sized to threadably receive the internal riser joint and providing the attachment point for the flange on the tapered riser joint.
U.S. Pat. No. 7,013,824 discloses a riser centralizer for transferring lateral loads from the riser to a platform hull with a keel centralizer mounted on a keel joint. The keel centralizer is received within a keel guide sleeve secured in a support mounted at the lower end of the platform hull. The keel centralizer includes a nonmetallic composite bearing ring having a radiused peripheral profile for minimizing contact stresses between the keel centralizer and the keel guide sleeve in extremes of riser and platform motion. The internal surface of the keel guide sleeve is clad with a corrosion resistant alloy and coated with a wear resistant ceramic rich coating. In one aspect, there is keel centralizer that includes: a. a flat keel centralizer body having a central bore extending through said body; b. the keel centralizer body including a circumferential flange member defining the perimeter thereof; c. at least one opening extending through the keel centralizer body; d) a bearing ring mounted on the flange member; and e) a keel sleeve mounted in a keel support frame, the keel sleeve being adapted for slidably receiving the keel centralizer body, and wherein the keel sleeve is clad with a corrosion resistant material.
U.S. Pat. No. 6,746,182 discloses keel joint assemblies that permit a degree of rotational movement of a riser within the keel of a floating vessel and greatly reduce the amount of stress and strain that is placed upon the riser, as well. Keel joint assemblies described provide a limiting joint between the riser and the keel opening that permits some angular rotation of the riser with respect to the floating vessel. Additionally, the limiting joint permits the riser to move upwardly and downwardly within the keel opening, but centralizes the riser with respect to the keel opening so that the riser cannot move horizontally with respect to the keel opening. In certain embodiments, the limiting joint is provided by a single annular joint that allows that riser to move angularly with respect to the can. In some embodiments, the keel joint assembly incorporates a cylindrical stiffening can that radially surrounds a portion of the riser and is disposed within the keel opening. In these embodiments, a flexible joint is provided between the can and the riser. Supports or guides may be used to retain the can within the keel opening. In one aspect, in floating platform, there is: a hull having a bottom and a deck spaced above the bottom; a riser opening extending generally vertically through the hull from the bottom to the deck; a riser extending through the riser opening; a landing profile in the riser opening adjacent to the bottom of the hull; a guide sleeve having an engagement profile that lands and locks on the landing profile for movement with the hull; and a collar being located with the guide sleeve and having a flex member having a central passage through which the riser extends, the flex member being supported by the guide sleeve adjacent to the bottom of the hull, the flex member being movable axially relative to an axis of the riser and allowing angular movement of the guide sleeve relative to the riser.
U.S. Pat. No. 6,422,791 discloses an attachment that extends between an outer sleeve and an inner riser pipe where the pipe penetrates the keel of a platform. In one version, the attachment is a conically-shaped with a small diameter ring that engages the riser pipe and a large diameter ring that engages the outer sleeve. This attachment has elements that are very flexible in bending but relatively stiff and strong in axial load. Other versions include flat rings where lateral load is taken directly into tension and compression in the beams, allowing for relatively high lateral load transfer. Both the conically-shaped attachment and the flat ring have a number of variations that provide low bending stiffness but high axial stiffness of the elements. Depending on whether resistance to axial loads, lateral loads, or resistance to combination of both loads is desired, the attachment and the flat ring may be used alone or in combination. Other variations of the device provide two opposing conical shaped attachments or a conical and flat ring attachment installed together to provide load capability in both axial and lateral directions while still providing angular flexibility. In one aspect there is a riser joint for a riser extending between a floating vessel and a sea floor, the riser joint including: a tubular member having an axis; a sleeve surrounding a portion of the tubular member and having an upper end, a lower end, and a sleeve axis that substantially aligns with the axis of the tubular member; a metal upper element adjacent to the upper end of the sleeve, and having a first portion mounted to the sleeve, and a second portion mounted to the tubular member, wherein the first and second portions of the upper element are axially spaced apart; a metal lower element adjacent to the lower end of the sleeve, and having a first portion mounted to the sleeve, and a second portion mounted to the tubular member, wherein the first and second portions of the lower element are axially spaced apart; and wherein the upper and lower elements have apertures therein between the first and second portions to allow angular and radial flexibility of the tubular member relative to the sleeve and resist axial motion of the tubular member relative to the sleeve.
U.S. Pat. No. 4,634,314 discloses composite marine riser system which, in certain aspects, include tubular marine riser sections formed by a wound or woven filament-resin matrix tubular body having a modulus of elasticity in tension of about 27,000,000 psi or greater utilizing carbon or boron for the filament material. The riser sections may be provided with end couplings secured to the tubular body for forming a riser system wherein the tensile strength or load bearing capacity of each section and its hydrostatic collapse resistance may be selectively determined by its position in the riser system. The riser sections may be provided with cylindrical collapse resisting ribs defining spaced buoyancy chambers filled with low density material contained by an outer shell formed of a glass or aramid fiber-resin matrix composite having a lower modulus of elasticity in tension than the primary load bearing tubular member. An inner abrasion and fluid impervious sleeve is disposed within the tubular body but is not bonded thereto. Multiple conduit riser sections may be utilized as anchoring members for a floating platform and the like. In one aspect there is a marine riser for use in drilling or production of hydrocarbons from a subsea formation including: elongated tubular body means constructed of a composite of elongated filaments of a material in a resin matrix having a modulus of elasticity not less than about 27.times.10.sup.6 psi, said filaments being bonded in a resin matrix to form a load bearing member of the tubular body means having an elastic elongation strain characteristic under stress in tension not substantially greater than steel; and coupling means at opposite ends of the tubular body means for coupling the riser to a member for transmission of tensile loads through the tubular body means between said ends.